Methods of and means for the estimation of small quantities of gaseous impurities inrare gases



METHODS OF AND MEANS FOR THE ESTIMATION OF SMALL QUANTITIES 0F GASEOUS, IMPURITIES 1N RARE GASES Filed Sept. 21, 1955 Dec. 8, 1959 .1. DUNDERDALE ETAL 2,916,693

INVENTOR ALFRED HORACE COCKETT JOHN DUNDERDALE ATTORNEY United States Patent METHODS OF AND MEANS FOR THE ESTIMA- TION OF SMALL QUANTITIES OF GASEOUS IlVIPURIT IES IN RARE GASES John Dunderdale, Sutton, and Alfred Horace Cockett,

Ruislip, England, assignors to The British Oxygen Company Limited, a British company Application September 21, 1955, Serial No, 535,663

3 Claims. 01. 324-33 I V This invention relates to methods of and means for estimating small quantities of gaseous impurities in rare minor amounts of impurity is not harmful, butin certain applications the rare gasis required in a high state of purity. For example, argon intended for use as shield- ."ing gas in. gas shielded arc welding must have a relatively low nitrogen content and similarly there is a definite upper limit to the amount of methane tolerable in helium intended for use as a shielding gas. Other common con- .taminants present in small quantities in commercial rare gases are air and carbon dioxide.

The rare gases are relatively inert and where the contaminant is also inert, as in the case of nitrogen,.it will be apparent that only methods based on physical effects are likely to be practicable for the estimation ofthe contaminant. Even where the contaminant is relatively reactive chemically, physical methods are still preferable on the grounds of speed and simplicity. The present invention utilises the well known eifect of traces of impurity on the colour and/or brightness of a silent electric discharge in the rare gas at low pressure. Thus, in gthe case of argon/nitrogen, the discharge in pure argon is blue,

but*the addition of nitrogen causes it to become more pink. This phenomenon has previously been used to obtain a reliable decision as to whether argon is spectrally pure by visual examination of the colour of the discharge ,therein. It has now been found that this phenomenon can also be made the basis of a quantitative estimation of small quantities of gaseous impurities in rare gases.

According to one aspect of the present invention, a method of estimating small quantities of gaseous impurities in a rare gas comprises passing a silent electric discharge through the impure rare gas at low pressure and measuring photoelectrically the variation in colour and/ or brightness of the discharge caused by variation in the impurity content of the rare gas.

The maximum impurity content which can be successfully estimated by the method of the present invention will vary with each impurity and each rare gas. Thus, for example, for nitrogen in argon the upper limit is about 3000 volumes per million, since above this value, the

, sensitivity of the method becomes low and concentrations of nitrogen above 5000 volumes per million extinguish the discharge. For nitrogen in helium, methane. in argon or helium and air in argon or helium, the upper limit is about 5000 volumes per million, while for carbon dioxide in argon or helium, it is about 1000 volumes per million. The variation of colour or brightness will also vary with the components of the gaseous mixture concerned.

As stated above, the colour change caused by the addition of nitrogen to argon is from blue to pink. In the case of nitrogen in helium, the discharge in pure helium is straw yellow, the addition of nitrogen causing the discharge to become more pink. In the case of methane in argon or helium, the colour of the discharge is a brilliant light blue which varies in intensity with the concentration of methane. The addition of air to argon or helium causes the discharge to become more pink, while with carbon dioxide the discharge is a brilliant greenish-blue which varies in intensity with the carbon dioxide content. It is normally desirable to dry the rare gas prior to entering the silent electric discharge tube. 7

According to another aspect of the invention, apparatus for the estimation of small quantities of gaseous impurities in a rare gas comprises an electrical discharge tube, means for passing the impure rare gas at low pressure through the discharge tube and photoelectric means for measuring the variation in colour and/or brightness of the discharge caused by variation in the impurity content of the rare gas.

The discharge tube used may be of any convenient shape, but a preferred tube is in the form of a glass tube bent to form a series of sections of reasonable length, so as to give a long gas path without becoming unwieldy. For example, a glass tube of inch diameter giving a discharge path of 48 inches may conveniently be doubled on itself to form a bundle of six sections, each 8 inches in length. Preferably, all the sections in the bundle are arranged to lie in a plane parallel to that of the active surface of the photoelectric means so that the latter views a sheet of discharge. The electrodes are preferably in the form of hollow metal cylinders closed at the end, to which electric leads are attached and having perforations to permit free circulation of gas.

The photoelectric means comprises one or more photoelectric cells, the number required depending on the size of cell available. The discharge tube and photoelectric cells are contained within a light-tight casing, which may be lined with a reflective or diffusive material to increase the intensity of illumination of the photocells and therefore the sensitivity of the apparatus. The output from the cell or cells is measured by a suitable instrument, which after calibration of the apparatus may be fitted with a scale ruled directly with volumes per rriillion of impurity so as to make the apparatus direct-reading. The sensitivity of the measuring instrument may be adjusted by means of variable shunts so as to permit a full-scale deffection of the pointer for various ranges of impurity content. These variable shunts may also be used to compensate for changes in the electrical characteristics of the photocells. In order to keep the scale as linear as possible, resistance of the measuring instrument and of H1116 shunts should all be low, for example, less than 0 ms.

One means for passing the impure gas through the discharge tube, which forms a part of the apparatus according to the present invention is illustrated in Fig. 1 of the accompanying drawing. It comprises a gas inlet 1, connected through a fine adjustment valve 2 with a branched tube 3. One branch of tube 3 is connected through a lute 4 with a rotameter type fiowmeter 5 and the other through a capillary 6, a stop valve 7 and a liquid trap 8 with one end of the discharge tube 9. The other end of the discharge tube 9 is connected by tube 10 through a stop valve 11, and a fine adjustment valve 12 to a vacuum pump (not shown). A vacuum gauge 13 is connected by tube 14 to tube 10 between valves 11 and 12.

In operation, the inlet 1 is connected to a source of the impure rare gas at a pressure above atmospheric. The apparatus is then started up by opening fine adjustment valve 2 (stop valve 7 being closed). The gas then passes to waste through the lute 4 and rotameter 5. The vacuum pump is then started and valve 7 opened. The gas stream entering the gas inlet 1, after passing through the fine adjustment valve 2 divides into two streams, one passing through the lute 4 to the fiowmeter 5 and thence to waste, and the other through the capillary 6, stop valve 7 and trap 8 to the discharge tube 9. The lute 4 is intended to maintain a constant pressure head on the gas passing through the flow control capilllary 6 to the discharge tube 9. The flowmeter 5 facilitates control of the excess gas passing to waste. The pressure in the discharge tube 9 is controlled by adjustment of the fine adjustment valves 2 and '12 and is measured by the vacuum gauge 13.

The liquid trap 8, which is desirable but may not be essential in certain analyses, and which may be cooled by immersion in liquid oxygen or nitrogen or in a solid carbon dioxide/trichlorethylene freezing mixture, serves to prevent traces of water vapor or like easily-condensible substances from being carried into the discharge tube. It has the further advantage that it slightly increases the colouration of the discharge and therefore the overall sensitivity of the apparatus, and it also promotes initiation of the discharge when a voltage is first applied to the tube 9.

The means described are intended to operate with a source of gas at a pressure above atmospheric. It will be understood that the method can readily be adapted for analysing rare gases at a pressure below atmospheric but above that required in the discharge tube 9.

In order to obtain a true reading on the measuring instrument, various compensations are required. In the first place, when a pure rare gas is passed through the discharge tube, a current is indicated by the measuring instrument. This can conveniently be neutralised by a further photoelectric cell or cells, the exact balance being obtained by adjusting a resistance in series therewith. These cells must have a diiferent spectral sensitivity curve from that of the main measuring photocells.

It is found that the sensitivity of the apparatus falls with increasing temperature, the coefficient in terms of volumes per million per degree centigrade increasing with increasing impurity concentrations. This coefficient is almost proportional to the reading of the measuring instrument and therefore, in theory, compensation could be effected by including in series with the measuring instrument a resistance with a high temperature coeflicient. However, in practice, apart from its undesirable effect on the linearity of the scale and on the sensitivity, it is difficult to find a material which has the requisite combination of low resistance and high temperature coeflicient. It is preferred, therefore, to effect temperature compensation by the inclusion of an extra circuit. A thermistor is used to measure the temperature and is connected in a Wheatstone bridge with three fixed resistance arms. The magnitude of the out-of-balance current is controlled by the temperature of the thermistor and also by the applied to the bridge. The latter is varied in accordance with the concentration of impurity in the gas. The out-of-balance current is added algebraically to the photoelectric current. The bridge has a relatively high resistance and therefore does not affect the sensitivity of linearity of the apparatus.

It will be appreciated that the measuring instrument will need recalibration for each different combination of gases and that, similarly, adjustments will have to be made in the various compensations.

When the impurity to be estimated is present in the rare gas in a very small concentration, the sensitivity of the method of the present invention can in certain cases be very considerably increased by adding a small known amount of another impurity. This has the unexpected effect of increasing the sensitivity by considerably more than the algebraic sum of the concentrations of the two impurities. For example, when estimating 10 volumes per million of nitrogen in argon, the addition of 10 volumes per million of carbon dioxide increases the sensitivity and an apparent total impurity concentration of about 30 volumes per million is observed.

Figure 2 is a diagram of the circuit used for the photoelectric means and incorporating various compensating circuits.

Referring to Figure 2, the measuring photocell E is connected in series with a measuring instrument M, adapted to measure the output of the photocell E. The measuring instrument M is provided with two variable shunts P and P either of which may be put in parallel with the instrument by means of a switch S When a pure rare gas is passed through the discharge tube, it is found that a current is indicated by the measuring instrument, which as shown in Figure 2, is neutralised by the output of a further photocell G, connected in parallel with the measuring photocell E in such manner that its output is in opposition to that of photocell E. The photocell G is selected to have a spectral range such that its output is not substantially affected by the presence of the impurity in the rare gas. The exact balancing of the outputs of the two photocells E and G when pure rare gas passes through the discharge tube is obtained-by means of a variable resistance R in series with photocell G.

It is also found that the sensitivity of the apparatus falls with increasing temperature, the coefiicient in terms of volumes per million per degree centigrade increasing with increasing impurity concentration. This coefiicient is almost proportional to the reading of the measuring instrument and therefore, in theory, compensation could be effected by including in series with the measuring instrument a resistance with a high temperature coefficient. However, in practice, apart from its undesirable elfect on the linearity of the scale of the measuring instrument and on the sensitivity of the apparatus, it is difiicult to find a material which has the requisite combination of low resistance and high temperature coefficient. It is preferred, therefore, to effect temperature compensation by the inclusion of an additional circuit as shown in Figure 2.

A thermistor T is used to measure the temperature and is connected in series with a variable resistance R, as one arm of a Wheatstone bridge, the other three arms of which are fixed resistances R R and R The variable resistance R is used to balance the bridge at the predetermined temperature for which the measuring instrument M is calibrated.

The bridge is provided with a source of E.M.F., V, whose output is controlled manually by a potentiometer P, the application of this E.M.F. being controlled by a switch S The bridge is connected across the measuring instrument M so that the out-of-balance current is added algebraically to the output of the photocell E as measured by the instrument M.

In operation, the concentration of impurity is measured with the switch S open, the potentiometer P is then adjusted in accordance with the measured concentration, and switch S is then closed and the impurity concentration accurately measured.

The bridge has a relatively high resistance and therefore does not affect the sensitivity of the apparatus.

We claim:

1. Apparatus for the estimation of small quantities of a gaseous impurity in a rare gas comprising an electrical discharge tube; a pair of electrodes mounted within said discharge tube; a source of electrical power connected between said electrodes; a source of the impure rare gas connected to one end of said discharge tube; a vacuum pump connected to the other end of said discharge tube; fine adjustment valve means connected between said source of impure rare gas and said one end of said discharge tube; at least one photoelectric cell positioned for illumination by light from said discharge tube; and

a measuring instrument in circuit with said photoelectric cell to measure the output thereof; and temperature compensating means comprising a thermistor connected in a Wheatstone bridge having three fixed resistance arms; means for applying an to said bridge; means for varying said in accordance with the concentration of impurity in said gas as measured Without temperature compensation of the apparatus; said bridge being connected to said measuring instrument in such manner that the reading shown thereby is a measure of the algebraic sum of the output of said photoelectric cell and of the out-of-balance current through said bridge.

2. Apparatus for the estimation of small quantities of a gaseous impurity in a rare gas comprising an electrical discharge tube; a pair of electrodes mounted within said discharge tube; a source of electrical power connected between said electrodes; means for passing the impure gas at low pressure through said discharge tube; at least one photoelectric cell positioned for illumination by light from said discharge tube; at least one additional photoelectric cell positioned for illumination by light from said discharge tube and having a spectral range such that it is not responsive to variations in the light from said discharge tube caused by the presence of the gaseous impurity; and a measuring instrument in circuit with said first-mentioned photoelectric cell and said additional photoelectric cell to measure the difference between the outputs of the said cells.

3. Apparatus for the estimation of small quantities of a gaseous impurity in a rare gas comprising an electrical discharge tube; a pair of electrodes mounted within said discharge tube; a source of electrical power connected between said electrodes; a source of the impure rare gas connected to one end of said discharge tube; a vacuum pump connected to the other end of said discharge tube; fine adjustment valve means connected between said source of impure rare gas and said one end of said discharge tube; at least one photoelectric cell positioned for illumination by light from said discharge tube; at least one additional photoelectric cell positioned for illumination by light from said discharge tube and having a spectral range such that it is not responsive to varia tions in the light from said discharge tube caused by the presence of the gaseous impurity; and a measuring instrument in circuit with said first-mentioned photoelectric cell and said additional photoelectric cell to measure the difference between the outputs of the said cells.

References Cited in the file of this patent UNITED STATES PATENTS 1,379,266 Keeler May 24, 1921 2,370,475 Lemmers Feb. 27, 1945 2,544,078 Glassbrook Mar. 6, 1951 2,582,647 Morgan Ian. 15, 1952 2,640,870 Seitz June 2, 1953 2,654,051 Kenty Sept. 29, 1953 2,734,628 Schlayer Feb. 14, 1956 

1. APPARATUS FOR THE ESTIMATION OF SMALL QUANTITIES OF A GASEOUS IMPURITY IN A RARE GAS COMPRISING AN ELECTRICAL DISCHARGE TUBE; A PAIR OF ELECTRODES MOUNTED WITHIN SAID DISCHARGE TUBE; A SOURCE OF ELECTRICAL POWER CONNECTED BETWEEN SAID ELECTRODES: A SOURCE OF THE IMPURE RARE GAS CONNECTED TO ONE END OF SAID DISCHARGE TUBE; A VACUUM PUMP CONNECTED TO THE OTHER END OF SAID DISCHARGE TUBE; FINE ADJUSTMENT VALVE MEANS CONNECTED BETWEEN SAID SOURCE F IMPURE RARE GAS AND SAID NE END OF SAID DISCHARGE TUBE; AT LEAST ONE PHOTOELECTRIC CELL POSITIONED FOR ILUMINATION BY LIGHT FROM SAID DISCHARGE TUBE; AND A MEASURING INSTRUMENT IN CIRCUIT WITH SAID PHOTOELECTRIC CELL TO MEASURE THE OUTPUT THEREOF; AND TEMPERATURE COMPENSATING MEANS COMPRISING A THERMISTOR CONNECTED IN A WHEATSTONE BRIDGE HAVING THREE FIXED RESISTANCE ARMS; MEANS FOR APPLYING AN E.M.F. TO SAID BRIDGE; MEANS FOR VARYING SAID E.M.F. IN ACCORDANCE WITH THE CONCENTRATION OF IMPURITY IN SAID GAS AS MEASURED WITHOUT TEMPERATURE COMPENSATION OF THE APPARATUS; SAID BRIDGE BEING CONNECTED TO SAID MEASURING INSTRUMENT IN SUCH MANNER THAT THE READING SHOWN THEREBY IS A MEASURE OF THE ALGEBRAIC SUM OF THE OUTPUT OF SAID PHOTOELECTRIC CELL AND OF THE OUT-OF-BALANCE CURRENT THROUGH SAID BRIDGE. 